Guiding the believers to what they should employ in their fights against the idolators, Allah says,

[فَإِذَا لَقِيتُمُ الَّذِينَ كَفَرُواْ فَضَرْبَ الرِّقَابِ]

(So, when you meet those who disbelieve (in battle), smite their necks) which means, `when you fight against them, cut them down totally with your swords.'

[حَتَّى إِذَآ أَثْخَنتُمُوهُمْ]

(until you have fully defeated them,) meaning, `you have killed and utterly destroyed them.'

[فَشُدُّواْ الْوَثَاقَ]

(tighten their bonds.) `This is referring to the prisoners of war whom you have captured. Later on, after the war ends and the conflict has ceased, you have a choice in regard to the captives: You may either act graciously toward them by setting them free without charge, or free them for a ransom that you require from them.' It appears that this Ayah was revealed after the battle of Badr. At that time, Allah reproached the believers for sparing many of the enemy's soldiers, and holding too many captives in order to take ransom from them. So He said then:

[مَا كَانَ لِنَبِىٍّ أَن يَكُونَ لَهُ أَسْرَى حَتَّى يُثْخِنَ فِي الاٌّرْضِ تُرِيدُونَ عَرَضَ الدُّنْيَا وَاللَّهُ يُرِيدُ الاٌّخِرَةَ وَاللَّهُ عَزِيزٌ حَكِيمٌ - لَّوْلاَ كِتَـبٌ مِّنَ اللَّهِ سَبَقَ لَمَسَّكُمْ فِيمَآ أَخَذْتُمْ عَذَابٌ عَظِيمٌ ]

(It is not for a Prophet to have captives of war until he had made a great slaughter (among the enemies) in the land. You desire the commodities of this world, but Allah desires (for you) the Hereafter. Allah is Mighty and Wise. Were it not for a prior decree from Allah, a severe torment would have touched you for what you took.) (8:67-68) Allah's saying,

[حَتَّى تَضَعَ الْحَرْبُ أَوْزَارَهَا]

(...until the war lays down its burden.) Mujahid said: "Until `Isa bin Maryam (peace be upon him) descends.'' It seems as if he derived this opinion from the Prophet's saying,

«لَا تَزَالُ طَائِفَةٌ مِنْ أُمَّتِي ظَاهِرِينَ عَلَى الْحَقِّ حَتْى يُقَاتِلَ آخِرُهُمُ الدَّجَّال»

(There will always be a group of my Ummah victorious upon the truth, until the last of them fight against Ad-Dajjal.) Imam Ahmad recorded from Jubayr bin Nufayr who reported from Salamah bin Nufayl that he went to the Messenger of Allah and said, "I have let my horse go, and thrown down my weapon, for the war has ended. There is no more fighting.'' Then the Prophet said to him,

«الْانَ جَاءَ الْقِتَالُ، لَا تَزَالُ طَائِفَةٌ مِنْ أُمَّتِي ظَاهِرِينَ عَلَى النَّاسِ، يُزِيغُ اللهُ تَعَالَى قُلُوبَ أَقْوَام، فَيُقَاتِلُونَهُمْ وَيَرْزُقُهُمُ اللهُ مِنْهُمْ حَتْى يَأْتِيَ أَمْرُ اللهِ وَهُمْ عَلَى ذلِكَ، أَلَا إِنَّ عُقْرَ دَارِ الْمُؤْمِنِينَ الشَّامُ، وَالْخَيْلُ مَعْقُودٌ فِي نَوَاصِيهَا الْخَيْرُ إِلَى يَوْمِ الْقِيَامَة»

(Now the time of fighting has come. There will always be a group of my Ummah dominant over others. Allah will turn the hearts of some people away (from the truth), so they (that group) will fight against them, and Allah will bestow on them (war spoils) from them (the enemies) -- until Allah's command comes to pass while they are in that state. Verily, the center of the believers' abode is Ash-Sham. And goodness is tied around the horses' foreheads till the Day of Resurrection.) An-Nasa'i also recorded this narration. Allah then says,

[ذَلِكَ وَلَوْ يَشَآءُ اللَّهُ لاَنْتَصَرَ مِنْهُمْ]

(Thus, and had Allah so willed, He could have taken vengeance against them;) which means that had He so willed, He could have taken immediate vengeance against the disbelievers with a chastisement or exemplary punishment directly from Him.

[وَلَـكِن لِّيَبْلُوَ بَعْضَكُمْ بِبَعْضٍ]

(but (He lets you struggle) so as to test with one another.) meaning, He has ordered Jihad and fighting against the enemies in order to try you and test your affairs. Allah also expresses His wisdom by the legislation of Jihad in the following two Surahs, Al `Iman and At-Tawbah, in which He says,

[أَمْ حَسِبْتُمْ أَن تَدْخُلُواْ الْجَنَّةَ وَلَمَّا يَعْلَمِ اللَّهُ الَّذِينَ جَـهَدُواْ مِنكُمْ وَيَعْلَمَ الصَّـبِرِينَ ]

(Or did you think that you would enter the Paradise before Allah had made evident those of you who fight in His cause and made evident those who are steadfast) (3:142)

[قَـتِلُوهُمْ يُعَذِّبْهُمُ اللَّهُ بِأَيْدِيكُمْ وَيُخْزِهِمْ وَيَنْصُرْكُمْ عَلَيْهِمْ وَيَشْفِ صُدُورَ قَوْمٍ مُّؤْمِنِينَ - وَيُذْهِبْ غَيْظَ قُلُوبِهِمْ وَيَتُوبُ اللَّهُ عَلَى مَن يَشَآءُ وَاللَّهُ عَلِيمٌ حَكِيمٌ ]

(Fight them; Allah will then punish them by your hands, disgrace them, grant you victory over them, satisfy the breasts of a believing people, and remove the fury of their (the believers') hearts. Allah turns in forgiveness to whom He wills; Allah is Knowing and Wise.) ( 9:14-15)